Nanotubes-101 Presentation

The
intent of this page is to convey a
general understanding of what
Nanotubes
-CNTs) are, how they are
produced, their many unique and interesting properties, markets, and
applications.

History of Carbon
Nanotubes -CNTs

In 1980 we knew of
only three forms of carbon, namely diamond, graphite, and amorphous
carbon. Today we know there is a whole family of other forms of
carbon. The first to be discovered was the hollow, cage-like
buckminsterfullerene molecule - also known as
the buckyball,or the C60 fullerene. There are now thirty or more forms of fullerenes, and
also an extended family of linear molecules, carbon nanotubes.
C60
is the first spherical carbon molecule, with carbon atoms arranged
in a soccer ball shape.
In the structure there are 60 carbon atoms
and a number of five-membered rings isolated by six-membered rings.
The second, slightly elongated, spherical carbon molecule in the
same group resembles a rugby ball, has seventy carbon atoms and is
known as C70. C70’s structure has extra six-membered carbon rings,
but there are also a large number of other potential structures
containing the same number of carbon atoms. Their particular shapes
depend on whether five-membered rings are isolated or not, or
whether seven-membered rings are present. Many other forms of
fullerenes up to and beyond C120 have been characterized, and it is
possible to make other fullerene structures with five-membered rings
in different positions and sometimes adjoining one another.
[“Nanotechnology: Basic Science and Emerging Technologies”, M.
Wilson et al, (2002)]

The important fact for nanotechnology is that useful dopant atoms
can be placed inside the hollow fullerene ball.
Atoms contained
within the fullerene are said to be endohedral.Of course they can
also be bonded to fullerenes outside the ball as salts, if the
fullerene can gain electrons.
[“Nanotechnology: Basic Science and Emerging Technologies”, M.
Wilson et al, (2002)]

Endohedral fullerenes can be produced in which metal atoms are
captured within the fullerene cages. Theory shows that the maximum
electrical conductivity is to be expected for endohedral metal
atoms, which will transfer three electrons to the fullerene.
Fullerenes can be dispersed on the surface as a monolayer. That is,
there is only one layer of molecules, and they are said to be mono
dispersed. Provided fullerenes can be placed in very specific
locations, they may be aligned to form a fullerene wire. Systems
with appropriate material inside the fullerene ball are conducting
and are of particular interest because they can be deposited to
produce bead-like conducting circuits. Combining endohedrally doped
structures with non-doped structures changes the actual composition
of a fullerene wire, so that it may be tailored in-situ
during patterning. Hence within a single wire, insulating and
conducting regions may be precisely defined. One-dimensional
junction engineering becomes realistic with fullerenes.
[“Nanotechnology: Basic Science and Emerging Technologies”, M.
Wilson et al, (2002)]

Possibly more
important than C60 fullerenes are
Carbon Nanotubes (CNTs), which are related to
graphite. The molecular structure of graphite resembles stacked,
one-atom-thick sheets of chicken wire - a planar network of
interconnected hexagonal rings of carbon atoms. In conventional
graphite, the sheets of carbon are stacked on top of one another,
allowing them to easily slide over each other. That is why graphite
is not hard, but it feels greasy, and can be used as a lubricant.
When graphene sheets are rolled into a cylinder and their edges
joined, they form
Carbon Nanotubes (CNTs). Only the tangents of the graphitic planes
come into contact with each other, and hence their properties are
more like those of a molecule.
[“Nanotechnology: Basic
Science and Emerging Technologies”, M. Wilson et al, (2002)]

Carbon Nanotubes (CNTs) may
consist of one tube of graphite, a one-atom thick
single walled nanotubes (SWNTs)
a two atom thick double walled carbon nanotubes (DWNTs) or a number of
concentric tubes called multiwalled
nanotubes (MWNTs). When viewed with a transmission electron microscope these
tubes appear as planes. Whereas
single walled nanotubes (SWNTs) appear as
two planes, in multi walled nanotubes more than two planes are
observed, and can be seen as a series of parallel lines. There are
different types of Carbon Nanotubes (CNTs), because the graphitic sheets can be rolled
in different ways. The three types of Carbon Nanotubes (CNTs)
structure are Zigzag, Armchair, and Chiral. It is possible to recognize zigzag, armchair, and chiral Carbon Nanotubes (CNTs) just by following the pattern across the diameter of the tubes,
and analyzing their cross-sectional structure.
[“Nanotechnology: Basic Science and Emerging Technologies”, M.
Wilson et al, (2002)]

Multi walled nanotubes (MWNTs) can come in an even more complex array of forms, because
each concentric nanotube can have different
structures, and hence there are a variety of sequential
arrangements. The simplest sequence is when concentric layers are
identical but different in diameter. However, mixed variants are
possible, consisting of two or more types of concentric Carbon Nanotubes (CNTs)
arranged in different orders. These can have either regular layering
or random layering.
The structure of the carbon nanotube
(CNTs) influences its
properties- including electrical and thermal conductivity, density,
and lattice structure. Both type and diameter are important. The
wider the diameter of the carbon nanotube (CNTs), the more it behaves like
graphite. The narrower the diameter of the
Carbon Nanotubes (CNTs), the more its
intrinsic properties depends upon its specific type.

There are a number of methods of making
Carbon Nanotubes (CNTs) and
C60 Fullerenes.C60 Fullerenes were first observed after vaporizing graphite with a
short-pulse, high-power laser, however this was not a practical
method for making large quantities. Carbon Nanotubes (CNTs) have probably been around
for a lot longer than was first realized, and may have been made
during various carbon combustion and vapor deposition processes, but
electron microscopy at that time was not advanced enough to
distinguish them from other types of tubes. The first
method for
producing Carbon Nanotubes (CNTs) and
C60 Fullerenes in reasonable quantities – was by
applying an electric current across two carbonaceous electrodes in
an inert gas atmosphere. This method is called plasma arcing. It
involves the evaporation of one electrode as cations followed by
deposition at the other electrode. This plasma-based process is
analogous to the more familiar electroplating process in a liquid
medium. C60 Fullerenes and Carbon Nanotubes (CNTs) are formed by plasma arcing of
carbonaceous materials, particularly graphite.
The C 60 Fullerenes appear
in the soot that is formed, while the Carbon Nanotubes (CNTs) are deposited on the
opposing electrode. Another
method ofCarbon Nanotubes (CNTs) synthesis involves
plasma arcing in the presence of cobalt
with a 3% or greater
concentration. As noted above, the
Carbon Nanotubes (CNTs) product is a compact
cathode deposit of rod like morphology. However when cobalt is added
as a catalyst, the nature of the product changes to a web, with
strands of 1mm or so thickness that stretch from the cathode to the
walls of the reaction vessel. The mechanism by which cobalt changes
this process is unclear, however one possibility is that such metals
affect the local electric fields and hence the formation of the
five-membered rings. [“Nanotechnology: Basic Science and
Emerging Technologies”, M. Wilson et al, (2002)]

Arc Method Carbon
Nanotubes

The carbon
arc
discharge method, initially used for producing C60 fullerenes, is
the most common and perhaps easiest way to produce Carbon Nanotubes
(CNTs),as it is rather simple.
However, it is a technique that produces a complex mixture of
components, and requires further
purification - to separate the
Carbon Nanotubes (CNTs)
from the soot and the residual catalytic metals present in the crude
product. This method creates Carbon Nanotubes (CNTs) through arc-vaporization of two
carbon rods placed end to end, separated by approximately 1mm, in an
enclosure that is usually filled with inert gas at low pressure.
Recent investigations have shown that it is also possible to create
Carbon Nanotubes (CNTs) with the arc method in liquid nitrogen. A direct current of 50
to 100 A, driven by a potential difference of approximately 20 V,
creates a high temperature discharge between the two electrodes. The
discharge vaporizes the surface of one of the carbon electrodes, and
forms a small rod-shaped deposit on the other electrode. Producing
Carbon Nanotubes (CNTs) in high yield depends on the uniformity of the plasma arc, and
the temperature of the deposit forming on the carbon electrode.
[“Nanotechnology: Basic Science and Emerging Technologies”, M.
Wilson et al, (2002)]

Laser Method Carbon Nanotubes

In 1996 Carbon Nanotubes (CNTs) were
first synthesized using a dual-pulsed laser and achieved yields of
>70wt% purity. Samples were prepared by laser vaporization of
graphite rods with a 50:50 catalyst mixture of Cobalt and Nickel at
1200oC in flowing argon, followed by heat treatment in a vacuum at
1000oC to remove the C60 and other fullerenes. The initial laser
vaporization pulse was followed by a second pulse, to vaporize the
target more uniformly. The use of two successive laser pulses
minimizes the amount of carbon deposited as soot. The second laser
pulse breaks up the larger particles ablated by the first one, and
feeds them into the growing nanotube structure. The material
produced by this method appears as a mat of “ropes”, 10-20nm in
diameter and up to 100um or more in length. Each rope is found to
consist primarily of a bundle of single walled nanotubes, aligned
along a common axis. By varying the growth temperature, the catalyst
composition, and other process parameters, the average nanotube
diameter and size distribution can be varied. Arc-discharge and
laser vaporization are currently the principal methods for obtaining
small quantities of high quality CNTs. However, both methods suffer
from drawbacks. The first is that both methods involve evaporating
the carbon source, so it has been unclear how to scale up production
to the industrial level using these approaches. The second issue
relates to the fact that vaporization methods grow CNTs in highly
tangled forms, mixed with unwanted forms of carbon and/or metal
species. The Carbon Nanotubes (CNTs) thus produced are difficult to purify, manipulate,
and assemble for building
Carbon Nanotubes (CNTs)-device architectures for
practical applications. [“Nanotechnology: Basic Science and
Emerging Technologies”, M. Wilson et al, (2002)]

Ball Milling or Carbon
Nanotubes

Ball milling and
subsequent annealing is a simple method for the production of Carbon Nanotubes (CNTs).
Although it is well established that mechanical attrition of this
type can lead to fully nano porous microstructures, it was not until
a few years ago that Carbon Nanotubes (CNTs) and boron nitride
nanotubes (BNNTs) were produced
from these powders by thermal annealing. Essentially the method
consists of placing graphite powder into a stainless steel container
along with four hardened steel balls. The container is purged, and
argon is introduced. The milling is carried out at room temperature
for up to 150 hours. Following milling, the powder is annealed under
an inert gas flow at temperatures of 1400oC for six hours. The
mechanism of this process is not known, but it is thought that the
ball milling process forms nanotube nuclei, and the annealing
process activates nanotube growth. Research has shown that this
method produces more multi walled carbon nanotubes (MWNTs) and few
single walled
carbon nanotubes (SWNTs). [“Nanotechnology: Basic Science and Emerging
Technologies”, M. Wilson et al, (2002)]

Other
Methods of Carbon Nanotubes Production

Carbon Nanotubes (CNTs) can also be
produced by diffusion flame synthesis, electrolysis, use of solar
energy, heat treatment of a polymer, and low-temperature solid pyrolysis. In flame synthesis, combustion of a portion of the
hydrocarbon gas provides the elevated temperature required, with the
remaining fuel conveniently serving as the required hydrocarbon
reagent. Hence the flame constitutes an efficient source of both
energy and hydrocarbon raw material. Combustion synthesis has been
shown to be scalable for high-volume commercial production.
There has recently been new innovations in carbon nanotubes (CNTs)
synthesis allowing for the production of
Single Walled Carbon Nanotubes (SWNTs)
with no metal catalyst. The inventor of this process is
Jeanette Benavides.
[“Nanotechnology: Basic Science and Emerging Technologies”, M.
Wilsonet al,

Purification
of Carbon Nanotubes

Purification of
Carbon Nanotubes (CNTs) generally refers to the separation of Carbon Nanotubes (CNTs) from other entities,
such as carbon nanoparticles, amorphous carbon, residual catalyst,
and other unwanted species. The classic chemical techniques for
purification have been tried, but they have not been found to be
effective in removing the undesirable impurities. Three basic
methods have been used with varying degrees of success, namely
gas-phase, liquid-phase, and intercalation methods.

Generally, a
centrifugal separation is necessary to concentrate the
single walled carbon nanotubes
(SWNTs) in low-yield soot before the micro filtration operation,
since the nanoparticles easily contaminate membrane filters. The
advantage of this method is that unwanted nanoparticles and
amorphous carbon are removed simultaneously and the Carbon Nanotubes (CNTs) are not
chemically modified. However 2-3 mol nitric acid is useful for
chemically removing impurities.

It is now possible
to cut Carbon Nanotubes (CNTs) into smaller segments, by extended sonication in
concentrated acid mixtures or by using an extrusion system. The resulting Carbon Nanotubes (CNTs) form a colloidal
suspension in solvents. They can be deposited on substrates, or
further manipulated in solution, and can have many different
functional groups attached to the ends and sides of the Carbon
Nanotubes (CNTs).
[“Nanotechnology: Basic Science and Emerging Technologies”, M.
Wilsonet al,

Gas Phase
Carbon Nanotubes Purification:

The first successful
technique for purification of
Carbon Nanotubes (CNTs) was developed by Thomas
Ebbesen and coworkers. Following the demonstration that
Carbon Nanotubes (CNTs)
could be selectively attached by oxidizing gases these workers
realized that nanoparticles, with their defect rich structures might
be oxidised more readily than the relatively perfect
Carbon Nanotubes (CNTs). They
found that a significant relative enrichment of
Carbon Nanotubes (CNTs) could be
achieved this way, but only at the expense of losing the majority of
the original sample. [“Carbon
Nanotubes”,T. W. Ebbesen, Ann.
Rev. Mater. Sci.24, 235 (1994); Physics Today381, 678 (1996)]

A new gas-phase
method has been developed at the NASA Glenn Research Center to
purify gram-scale quantities of
single walled carbon nanotubes (SWNTs).
This method, a modification of a gas-phase purification technique
previously reported by Smalley and others, uses a combination of
high-temperature oxidations and repeated extractions with nitric and
hydrochloric acid. This improved procedure significantly reduces the
amount of impurities such as residual catalyst, and non-nanotube
forms of carbon within the Carbon Nanotubes (CNTs), increasing their stability
significantly.

It is important to keep
the Carbon Nanotubes (CNTs) well-separated in solution, so the Carbon Nanotubes (CNTs) are typically
dispersed using a surfactant prior to the last stage of separation.

Intercalation
Carbon Nanotubes Purification:

An alternative
approach
to purifying multi walled carbon nanotubes (MWNTs)was introduced in 1994 by a
Japanese research group. This technique made use of the fact that nanoparticles and other graphitic contaminants have relatively
“open” structures and can therefore be more readily intercalated
with a variety of materials that can close carbon nanotubes (CNTs). By
intercalating with copper chloride, and then reducing this to
metallic copper, the research group was able to preferentially
oxidize the nanoparticles away, using copper as an oxidation
catalyst. Since 1994, this has become a popular method for
purification of
Carbon Nanotubes (CNTs).
“The first stage is to immerse the crude cathodic deposit in a molten copper chloride and potassium chloride
mixture at 400oC and leave it for one week. The product of this
treatment, which contains intercalated nanoparticles and graphitic
fragments, is then washed in ion exchanged water to remove excess
copper chloride and potassium chloride. In order to reduce the
intercalated copper chloride-potassium chloride metal, the washed
product is slowly heated to 500oC in a mixture of Helium and
hydrogen and held at this temperature for 1 hour. Finally, the
material is oxidized in flowing air at a rate of 10oC/min to a
temperature of 555oC. Samples of cathodic soot which have been
treated this way consist almost entirely of carbon nanotubes (CNTs). A
disadvantage of this method is that some amount of nanotubes are
inevitably lost in the oxidation stage, and the final material may
be contaminated with residues of intercalates. A similar
purification technique, which involves intercalation with bromine
followed by oxidation, has also been described. [“Carbon
Nanotubes and Related Structures : New Materials for the
Twenty-first Century”, P. F. Harris, Cambridge University Press
(1999) ISBN 0-521-55446-2 page 49]

Dispersion of Carbon Nanotubes

The
Carbon Nanotubes (CNTs) solution is composed of either
Single Walled Carbon Nanotubes (SWNTs),
Double Walled Carbon Nanotubes (DWNTs) or
Multi Walled Carbon Nanotubes (MWNTs), PVP (or other surfactant), and water, in the proportions of
10 parts Carbon Nanotubes (CNTs): ~1-2 parts PVP: 2,000 parts water or other solvents.
The required dispersion (sonication) time is ~2 to 8 minutes with an
interruption of 10 seconds every 30 seconds at full or high
amplitude. If the power of your ultrasonic equipment is less than
that of the SONICS VCX750 unit then you must prolong the carbon
nanotubes sonication
time accordingly. For Dispersing Carbon Nanotubes (CNTs), we recommend a
pulsed sonication method for 40 minutes at reduced amplitude.
We typically pulse our sonicator on for 30 seconds & off for 10
seconds, then we repeat that step until we have reached the desired
sonication time. We have found that
to
effectively disperse Single Walled Carbon Nanotubes (SWNTs) that we need to set the amplitude at 40%
for much longer times to break apart the Van der Waals physical
bonds which make the Single
Walled Carbon Nanotubes (SWNTs) agglomerate into bundles. Since
Single Walled Carbon Nanotubes
(SWNTs)
are such a fine particle, the agglomerated bundles are harder to
disperse.

Although both probe
style and bath style ultrasonic systems can be used for
dispersing Carbon Nanotubes(CNTs),
it is widely believed that the probe style ultrasonic systems work
better for dispersing
Carbon Nanotubes
(CNTs). It is also widely known that
adding a dispersing reagent (surfactant) into the solution will
accelerate the dispersion effect. The reagent Polyvinyl Pyrrolidone
(PVP) is a good dispersion agent. Some people like to use the
reagent Sodium Dodecyl Benzene Sulfonate (SDBS) or Poly Vinyl
Alcohol (PVA) as well. We have found that the dispersing reagent
and proportions listed above do change when using different
solvents. When working on a new dispersion in IPA, Acetone, or any
solvents other than DI water, we usually start with 500mg Carbon Nanotubes
(CNTs), 125
mgs dispersing reagent, into 500 mls of solvent and use the
ultrasonic process detailed above. In our experience, much less
reagent is used for dispersing in DI water than other solvents. We
believe this is due to the high polarity of water compared to other
solvents. Typically, it is a question of chemistry to achieve a
stable dispersion. A stable dispersion will last for days, weeks,
or months with little to no settling.

In some applications,
achieving a stable dispersion can require other agents in the
solution to prevent the Carbon
Nanotubes (CNTs) from falling out of solution over
time. Emulsifier T-60 (also known as Tween 60) is commonly used
with Di water or Isopropyl Alcohol. Organic Titanates can be used
with Acetone and Xylene. The specific application determines
whether these agents remain in the solution when further processing,
or if they need to be removed. Some organic titanates can be
removed by heating the solution above 2500C. The
addition of the OH and COOH functional groups assists the carbon
nanotubes (CNTs)
dispersing in DI water and other solvents as well as the chemical
bonding to other materials during further processing.

Functionalization of
Carbon Nanotubes

Pristine
Carbon Nanotubes
(CNTs)
are unfortunately insoluble in many liquids such as water, polymer
resins, and most solvents. Thus they are difficult to evenly
disperse in a liquid matrix such as epoxies and other polymers. This
complicates efforts to utilize the
Carbon Nanotubes
(CNTs) outstanding physical
properties in the manufacture of composite materials, as well as in
other practical applications which require preparation of uniform
mixtures of Carbon Nanotubes-CNTs
-CNTs with many different organic, inorganic, and
polymeric materials.

To make Carbon Nanotubes-CNTs
more easily dispersible in liquids, it is necessary to physically or
chemically attach certain molecules, or functional groups, to their
smooth sidewalls without significantly changing the
Carbon Nanotubes
(CNTs)
desirable properties. This process is called functionalization. The
production of robust composite materials requires strong covalent
chemical bonding between the filler particles and the polymer
matrix, rather than the much weaker van der Waals physical bonds
which occur if the Carbon
Nanotubes -CNTs are not properly functionalized.

Functionalization
methods such as chopping, oxidation, and “wrapping” of the Carbon Nanotubes-CNTs
(CNTs) in
certain polymers can create more active bonding sites on the surface
of the Carbon Nanotubes-CNTs
(CNTs). For biological uses, Carbon Nanotubes-CNTs
(CNTs) can be functionalized by
attaching biological molecules, such as lipids, proteins, biotins,
etc. to them. Then they can usefully mimic certain biological
functions, such as protein adsorption, and bind to DNA and drug
molecules. This would enable medially and commercially significant
applications such as gene therapy and drug delivery. In biochemical
and chemical applications such as the development of very specific
biosensors, molecules such as carboxylic acid (COOH), poly m-aminobenzoic
sulfonic acid (PABS), polyimide, Amines (NH2) and polyvinyl alcohol (PVA) have
been used to functionalize
Carbon Nanotubes
(CNTs), as have amino acid derivatives,
halogens, and compounds. Some types of functionalized Carbon Nanotubes-CNTs
(CNTs) are
soluble in water and other highly polar, aqueous solvents.

Properties of Carbon Nanotubes-CNTs

There are many useful
and unique properties of Carbon Nanotubes-CNTs and some of them are
detailed below.

The carbon atoms of
a single sheet of graphite form a planar honeycomb lattice, in which
each atom is connected via a strong chemical bond to three
neighboring atoms. Because of these strong bonds, the basal plane
elastic modulus of graphite is one of the largest of any known
material. For this reason,Carbon
Nanotubes (CNTs) are expected to be the ultimate
high-strength fibers.
Single walled carbon nanotubes (SWNTs) are stiffer than
steel, and are very resistant to damage from physical forces.
Pressing on the tip of a
Carbon Nanotubes
(CNTs) will cause it to bend, but without
damage to the tip. When the force is removed, the Carbon Nanotubes
(CNTs) returns
to its original state. This property makes Carbon Nanotubes
(CNTs) very useful as probe
tips for very high-resolution scanning probe microscopy.
Quantifying these effects has been rather difficult, and an exact
numerical value has not been agreed upon.

Many applications
of Carbon Nanotubes
(CNTs), such as in nanoscale molecular electronics, sensing and
actuating devices, or as reinforcing additive fibers in functional
composite materials, have been proposed. Reports of several recent
experiments on the preparation and mechanical characterization of
Carbon Nanotubes
(CNTs)-polymer composites have also appeared. These measurements
suggest modest enhancements in strength characteristics of
Carbon Nanotubes
(CNTs)-embedded
matrixes as compared to bare polymer matrixes. Preliminary
experiments and simulation studies on the thermal properties of Carbon Nanotubes
(CNTs)
show very high thermal conductivity. It is expected, therefore, that
Carbon Nanotubes
(CNTs) reinforcements in polymeric materials may also
significantly improve the thermal and thermomechanical properties of
the composites.

Field emission results
from the tunneling of electrons from a metal tip into vacuum, under
application of a strong electric field. The small diameter and high
aspect ratio of Carbon nanotubes (CNTs) is very favorable for field emission. Even for
moderate voltages, a strong electric field develops at the free end
of supported Carbon Nanotubes (CNTs) because of their sharpness. This was observed by
de Heer and co-workers at EPFL in 1995. He also immediately realized
that these field emitters must be superior to conventional electron
sources and might find their way into all kind of applications, most
importantly flat-panel displays. It is remarkable that after only
five years Samsung actually realized a very bright color display,
which will be shortly commercialized using this technology.
Studying the field emission properties of multi walled carbon nanotubes
(MWNTs),
Bonard and co-workers at EPFL observed that together with electrons,
light is emitted as well. This luminescence is induced by the
electron field emission, since it is not detected without applied
potential. This light emission occurs in the visible part of the
spectrum, and can sometimes be seen with the naked eye. [B.Q.
Wei, et al, Appl. Phys. Lett. 79 1172 (2001)].

Carbon
Nanotubes (CNTs) represent a
very small, high aspect ratio conductive additive for plastics of
all types. Their high aspect ratio means that a lower loading of
Carbon Nanotubes (CNTs) is needed compared to other conductive additives to achieve the
same electrical conductivity. This low loading preserves more of the
polymer resins’ toughness, especially at low temperatures, as well
as maintaining other key performance properties of the matrix
resin. Carbon Nanotubes (CNTs) have proven to be an excellent additive to impart
electrical conductivity in plastics. Their high aspect ratio, about
1000:1 imparts electrical conductivity at lower loadings, compared
to conventional additive materials such as carbon black, chopped
carbon fiber, or stainless steel fiber.

The large surface
area and high absorbency of Carbon Nanotubes (CNTs) make them ideal candidates for use
in air, gas, and water filtration. A lot of research is being done
in replacing activated charcoal with Carbon nanotubes (CNTs) in certain ultra high
purity applications.

The special nature
of carbon combined with
the molecular perfection of single-walled
carbon nanotubes (SWNTs) to endow them with exceptional material properties,such
as very high electrical and thermal conductivity, strength,
stiffness, and toughness. No other element in the periodic table
bonds to itself in an extended network with the strength of the
carbon-carbon bond. The delocalized pi-electron donated by each atom
is free to move about the entire structure, rather than remain with
its donor atom, giving rise to the first known molecule with
metallic-type electrical conductivity. Furthermore, the
high-frequency carbon-carbon bonds vibrations provide an intrinsic
thermal conductivity higher than even diamond. In most conventional
materials, however, the actual observed material properties -
strength, electrical conductivity, etc. - are degraded very
substantially by the occurrence of defects in their structure. For
example, high-strength steel typically fails at only about 1% of its
theoretical breaking strength. Carbon Nanotubes
(CNTs), however, achieve values very
close to their theoretical limits because of their molecular
perfection of structure.

This aspect is
part of the unique story of
Carbon Nanotubes
(CNTs). Carbon
Nanotubes (CNTs) are an example of true
nanotechnology: they are under 100 nanometers in diameter, but are
molecules that can be manipulated chemically and physically in very
useful ways. They open an incredible range of applications in
materials science, electronics, chemical processing, energy
management, and many other fields. Carbon Nanotubes
(CNTs) have extraordinary
electrical conductivity, heat conductivity, and mechanical
properties. They are probably the best electron field-emitter
possible. They are polymers of pure carbon and can be reacted and
manipulated using the well-known and the tremendously rich chemistry
of carbon. This provides opportunity to modify their structure, and
to optimize their solubility and dispersion. Very significantly,
Carbon Nanotubes
(CNTs) are molecularly perfect, which means that they are normally
free of property-degrading flaws in the
Carbon Nanotubes
(CNTs) structure. Their
material properties can therefore approach closely the very high
levels intrinsic to them. These extraordinary characteristics give
Carbon Nanotubes
(CNTs) potential in numerous applications.

Carbon Nanotubes
(CNTs) are the best
known field emitters of any material. This is understandable, given
their high electrical conductivity, and the incredible sharpness of
their tip. The smaller the tip’s radius of curvature, the more
concentrated the electric field will be, leading to increased field
emission. The sharpness of the tip also means that they emit at
especially low voltage, an important fact for building low-power
electrical devices that utilize this feature. Carbon Nanotubes
(CNTs) can carry an
astonishingly high current density. Furthermore, the current is
extremely stable. An immediate application of this behavior
receiving considerable interest is in field-emission flat-panel
displays. Instead of a single electron gun, as in a traditional
cathode ray tube display, in
Carbon Nanotubes
(CNTs)-based displays there is a separate Carbon Nanotubes
(CNTs) electron gun for each individual pixel in the display.
Their high current density, low turn-on and operating voltages, and
steady, long-lived behavior make Carbon Nanotubes
(CNTs) very attractive field emitters
in this application. Other applications utilizing the field-emission
characteristics of Carbon
Nanotubes (CNTs) include general types of low-voltage
cold-cathode lighting sources, lightning arrestors, and electron
microscope sources.

Much of the history of plastics over the last half-century has
involved their use as a replacement for metals. For structural
applications, plastics have made tremendous headway, but not where
electrical conductivity is required, because plastics are very good
electrical insulators. This deficiency is overcome by loading
plastics up with conductive fillers, such as carbon black and larger
graphite fibers. The loading required to provide the necessary
conductivity using conventional fillers is typically high, however,
resulting in heavy parts, and more importantly, plastic parts whose
structural properties are highly degraded. It is well-established
that the higher the aspect ratio of the filler particles, the lower
the loading required to achieve a given level of conductivity.

Carbon Nanotubes
(CNTs) are ideal in this sense, since they have the highest aspect
ratio of any carbon fiber. In addition, their natural tendency to
form ropes provides inherently very long conductive pathways even at
ultra-low loadings. Applications that exploit this behavior of Carbon Nanotubes
(CNTs)
include EMI/RFI shielding composites; coatings for enclosures,
gaskets, and other uses such as electrostatic dissipation;
antistatic materials, transparent conductive coatings; and
radar-absorbing materials for stealth applications.

A lot of automotive
plastics companies are using
Carbon Nanotubes
(CNTs) as well. Carbon
Nanotubes (CNTs) have been added
into the side mirror plastics on automobiles in the US since the
late 1990s. I have seen forecasts predicting that GM alone will
consume over 500 pounds of
Carbon Nanotubes
(CNTs) masterbatches in 2006 for using in
all areas of automotive plastics. Masterbatches normally contain 20
wt% Carbon Nanotubes
(CNTs)which are already very well dispersed. Manufacturers then
need to perform a “let down” or dilution procedure prior to using
the masterbatch in production

Carbon Nanotubes
(CNTs) have the
intrinsic characteristics desired in material used as electrodes in
batteries and capacitors, two technologies of rapidly increasing
importance. Carbon Nanotubes
(CNTs) have a tremendously high surface area, good
electrical conductivity, and very importantly, their linear geometry
makes their surface highly accessible to the electrolyte.

Research has shown
that Carbon Nanotubes
(CNTs) have the highest reversible capacity of any carbon
material for use in lithium ion batteries. [B. Gao, Chem.
Phys. Lett. 327, 69 (2000)].
In addition, Carbon Nanotubes
(CNTs) are
outstanding materials for super capacitor electrodes [R.Z.
Ma, et al., Science in China Series E-Technological Sciences
43 178 (2000)] and are
now being marketed for this application.Carbon
Nanotubes (CNTs)
also have applications in a variety of fuel cell components. They
have a number of properties,including high surface
area and thermal conductivity,which make them useful
as electrode catalyst supports in PEM fuel cells. Because of their
high electrical conductivity, they may also be used in gas diffusion
layers,as well
as current collectors. Carbon
Nanotubes (CNTs) high strength and toughness-to-weight
characteristics may also prove valuable as part of composite
components in fuel cells that are deployed in transport
applications, where durability is extremely important.
CheapTubes.com has a new product coming out that is a carbon
nanotubes (CNTs) based conductive additive specifically for Li Ion
Batteries.

The same properties
that make Carbon Nanotubes
(CNTs) attractive as conductive fillers for use in
electromagnetic shielding, ESD materials, etc., make them attractive
for electronics packaging and interconnection applications, such as
adhesives, potting compounds, coaxial cables, and other types of
connectors.

The idea of building electronic
circuits out of the essential building blocks of materials -
molecules - has seen a revival the past few years, and is a key
component of nanotechnology. In any electronic circuit, but
particularly as dimensions shrink to the nanoscale, the
interconnections between switches and other active devices become
increasingly important. Their geometry, electrical conductivity, and
ability to be precisely derived, make CNTs the ideal candidates for
the connections in molecular electronics. In addition, they have
been demonstrated as switches themselves.

There are already companies such
as Nantero from Woburn, MA that are already making
Carbon Nanotubes
(CNTs) based non-volitle
random access memory for PC’s. A lot of research is being done to
design Carbon Nanotubes
(CNTs) based transistors as well.

The record-setting
anisotropic thermal conductivity of Carbon Nanotubes
(CNTs) is enabling many
applications where heat needs to move from one place to another.
Such an application is found in electronics, particularly heat sinks
for chips used in advanced computing, where uncooled chips now
routinely reach over 100oC.The
technology for creating aligned structures and ribbons of Carbon Nanotubes
(CNTs)
[D.Walters, et al., Chem. Phys. Lett. 338, 14 (2001)]
is a step toward
realizing incredibly efficient heat conduits. In addition,
composites with Carbon
Nanotubes (CNTs) have been shown to dramatically increase their
bulk thermal conductivity, even at very small loadings.

The superior
properties of Carbon Nanotubes
(CNTs) are not limited to electrical and thermal
conductivities, but also include mechanical properties, such as
stiffness, toughness, and strength. These properties lead to a
wealth of applications exploiting them, including advanced
composites requiring high values of one or more of these properties.

A ceramic material
reinforced with carbon nanotubes (CNTs) has been made by materials
scientists at UC Davis. The new material is far tougher than
conventional ceramics, conducts electricity and can both conduct
heat and act as a thermal barrier, depending on the orientation of
the
Carbon Nanotubes
(CNTs). Ceramic materials are very hard and resistant to
heat and chemical attack, making them useful for applications such
as coating turbine blades, but they are also very brittle.

The researchers mixed powdered alumina (aluminum oxide) with 5 to 10
percent carbon nanotubes (CNTs) and a further 5 percent finely milled
niobium. The researchers treated the mixture with an electrical
pulse in a process called spark-plasma sintering. This process
consolidates ceramic powders more quickly and at lower temperatures
than conventional processes.

The new material has up to five times the fracture toughness --
resistance to cracking under stress -- of conventional alumina. The
material shows electrical conductivity seven times that of previous
ceramics made with Carbon Nanotubes
(CNTs). It also has interesting thermal
properties, conducting heat in one direction, along the alignment of
the
Carbon Nanotubes
(CNTs), but reflecting heat at right angles to the
Carbon Nanotubes
(CNTs),
making it an attractive material for thermal barrier coatings.

The
exploration of Carbon Nanotubes
(CNTs) in biomedical applications is just underway, but
has significant potential. Since a large part of the human body
consists of carbon, it is generally though of as a very
biocompatible material. Cells have been shown to grow on Carbon Nanotubes
(CNTs), so
they appear to have no toxic effect. The cells also do not adhere to
the Carbon Nanotubes
(CNTs), potentially giving rise to applications such as coatings
for prosthetics and surgical implants. The ability to functionalize
the sidewalls of
Carbon Nanotubes
(CNTs) also leads to biomedical applications such as
vascular stents, and neuron growth and regeneration. It has also
been shown that a single strand of DNA can be bonded to a
Carbon Nanotubes
(CNTs,
which can then be successfully inserted into a cell; this has
potential applications in gene therapy.

Many researchers and corporations have already developed
Carbon Nanotubes
(CNTs) based
air and water filtration devices. It has been reported that these
filters can not only block the smallest particles but also kill most
bacteria. This is another area where Carbon Nanotubes
(CNTs) have already been
commercialized and products are on the market now. Someday Carbon Nanotubes
(CNTs)
may
be used to filter other liquids such as fuels and lubricants as
well.

A lot of research is being
done in the development of Carbon Nanotubes
(CNTs) based air and gas filtration.
Filtration has been shown to be another area where it is cost
effective to use
Carbon Nanotubes
(CNTs) already. The research I’ve seen suggests that
1 gram of Multi Walled Carbon Nanotubes (MWNTs) can be dispersed onto 1 sq ft of filter media.
Manufacturers can get their cost down to $0.95 cents per gram of
purified Multi Walled Carbon Nanotubes (MWNTs) when purchasing ton quantities.

Some commercial products on
the market today utilizing Carbon Nanotubes
(CNTs) include stain resistant textiles, Carbon Nanotubes
(CNTs) reinforced tennis rackets and baseball bats. Companies like
Kraft foods are heavily funding cnt based plastic packaging. Food
will stay fresh longer if the packaging is less permeable to
atmosphere. Coors Brewing company has developed new plastic beer
bottles that stay cold for longer periods of time. Samsung already
has Carbon Nanotubes
(CNTs) based flat panel displays on the market. A lot of companies
are looking forward to being able to produce transparent conductive
coatings and phase out ITO coatings. Samsung uses aligned
Carbon Nanotubes (SWNTs) in
the transparent conductive layer of their display manufacturing
process.

In closing, Carbon Nanotubes
(CNTs) have many
unique and desirable properties. Although many applications may
take significant investments of time and money to develop to reach
commercial viability, there are plenty of applications today in
which Carbon Nanotubes
(CNTs) add significant benefits to existing products with
relatively low implementation costs. Most of these applications are
in the polymer, composite materials, batteries, paints, plastics,
ceramics, automotive, and textiles industries.